FIG. 1 is a cross-sectional view of a portion of an image sensor according to the prior art. Image sensor 100 includes substrate 102 having photodetectors 104, threshold implant 106, well 108, lightly doped drain (LDD) 110, and heavy source/drain implant region 112 formed therein. The combination of well 108, LDD 110, and source/drain implant region 112 acts as a charge-to-voltage conversion region 114. Well 108 also operates as an anti-punch-through region between the LDD 110 or the source/drain implant region 112 to the photodetector 104.
Threshold implants 106 and well 108 are formed prior to the creation of transfer gates 116, while photodetectors 104 and LDD 110 are formed after the formation of transfer gates 116. Because photodetectors 104 and LDD 110 are created after transfer gates 116, photodetectors 104 and LDD 110 are self-aligned to the edges of transfer gates 116.
Source/drain implant region 112 is implanted into well 108 after sidewall spacers 118 are formed along the outside sides of transfer gates 116. Source/drain implant region 112 is formed when other source/drain implant regions, such as the source/drain implant regions of transistors, are formed in the image sensor. Source/drain implant region 112 is disposed underneath contact 120 and extends out from contact 120 into well 108. At least a portion of each transfer gate 116 is also implanted with dopants during the source/drain implant to form doped region 122. Doped region 122 advantageously affects the transfer gate 116 work function and increases the transfer gate conductivity.
The doping level for the source/drain implant regions, including source-drain implant region 112, is usually high to maintain high conductivity. Because the doping level is so high, the implant completely destroys the lattice structure and converts the single crystalline structure of well 108, LDD 110, and substrate layer 102 into an amorphous structure. Subsequent thermal processing steps are needed for the amorphous structure to rearrange back into a single crystalline structure. As technology advances, however, the post source/drain implant thermal budget is significantly reduced to reduce dopant lateral diffusion, so the implant damage may not be completely repaired by subsequent thermal processing.
One consequence of lattice damage or defects is a very high rate of dark current generation. Lattice damage also serves as a gettering site for metallic contaminants, which is undesirable because metallic contaminants are also known to generate very high dark current. To avoid damaging the substrate, the heavy source/drain implant in the charge-to-voltage conversion region is not performed during the fabrication of some image sensors. However, as described earlier, the heavy source/drain implant forms doped region 122 in transfer gates 116. Removing the doped region 122 will alter the transfer gate 116 work function and may adversely impact the electrical operation of the transfer gate.